1. Field of the Invention
The present invention relates to methods and apparatus for stabilization of a laser and more particularly concerns laser frequency stabilization that is thermally insensitive.
2. Description of Related Art
Frequency stabilization of a laser, such as a carbon dioxide waveguide laser, for example, may be accomplished by changing laser cavity length. Use of a piezoelectric crystal transducer that carries one of the cavity reflecting end mirrors is one arrangement for changing cavity length. The piezoelectric transducer is driven from the output of a dither stabilizing circuit provided in a feedback loop and having as an input an optical or laser output power signal provided by a power detector that optically senses the laser output power signature. Laser power signature is a graph of the cyclical variation of output power plotted against change in cavity length. As cavity length changes (is "scanned") through one power signature cycle, the output power varies through a plurality of peaks. Each full signature cycle (which may have several power peaks) occurs over a cavity length variation that depends on lasing wavelength. This variation may be a few micrometers for certain wavelengths. As change in cavity length continues, the signature cycle is repeated.
The optical signature detector sensor is mounted with the laser output optics to sample a portion of the output energy beam and provides an output power feedback signal that is fed to a stabilization circuit. The feedback stabilization electronics is a phase comparison circuit that effectively interprets the optical power signature sensed by the detector and compares the phase of change in output power with the phase of a dither signal generated in the feedback stabilization electronics or associated circuitry to provide a control signal for the piezoeleotric transducer. The latter, when energized by the output of the feedback circuit, changes cavity length, and therefore laser frequency, by an amount that tends to move the laser operating frequency back toward the peak of the laser gain curve upon which stabilization is centered.
The required use of an optical sensor or power detector in such frequency stabilization arrangements poses several problems. The detector requires additional optics which must be properly aligned and mounted adjacent the laser output optics. The detector adds weight and takes up space that may be quite valuable, particularly in lasers employed in military applications. Frequently some type of folding optics is required for operation of the power detector. Because the projected laser output beam must pass through the detector for power level sampling, the output beam is diminished and also may be distorted. The detector is expensive and delicate, thus adding to the cost and lack of laser reliability.
Problems with the optical detector have been recognized, as, for example, in the patent to Midavaine 4,694,458, which purports to offer a solution based on use of an RF (radio frequency) return signal in an RF excited CO.sub.2 laser. The RF return is fed to a stabilization circuit that is arranged and controlled to vary cavity length so as to maintain a substantially minimum magnitude of the RF return and thus keep the laser operating at the desired peak of the laser gain curve. The patent to Midavaine describes a system which recognizes the fact that the laser impedance will vary with its light intensity, to thereby perturb the match between the output impedance of the RF exciting source and the laser cavity impedance. An impedance matching circuit is adjusted to achieve minimum RF return at the selected operating frequency and temperature. Mismatch of impedances causes the reflected RF signal (the RF return) to vary from its minimum. As laser frequency or temperature varies, the mismatch increases and RF return increases from its minimum. The patentees utilize this variation of the RF return from its minimum as an input to a feedback circuit that controls a piezoelectric crystal to change cavity length, and thus change frequency.
What is not mentioned or even considered in the system the Midavaine et al patent is the fact that the described arrangement can work only if the laser has highly precise operating temperature stabilization. This is so because the magnitude of the RF return signal varies not only with light intensity within the laser, but also with laser operating temperature.
Laser operating temperature may be considered as an average temperature of the laser device. It is affected by a number of factors, including the heat exchanger or other external cooling system employed to limit variation of operating temperature, and a change of input power to the cavity. Input power changes with variation of either frequency or output power of the RF excitation source, or with drift of components of the impedance matching circuit employed to couple RF power to the laser cavity. These factors involve other than laser light induced temperature variation that cause a change of laser cavity impedance.
Laser cavity impedance variations due to variations in laser operating temperature must be distinguished from impedance variations induced by laser light in order to ensure frequency stabilization over a range of temperatures. This distinction is neither recognized nor accomplished in the Midavaine et al patent.
For the Midavaine system to work, the impedance variation due to operating temperature must be made effectively negligible. The system of Midavaine requires the laser operating temperature range to be substantially minimized, or effectively near zero. Furthermore, the Midavaine system cannot lock on a power peak that is outside of the range of the cavity length adjusting device. The Midavaine arrangement is basically only a laboratory device that can tolerate a temperature variation in the order no greater than about 10.degree.C. The Midavaine arrangement is not applicable to any real life or practical system that is subject to environmental temperature variation or the many tens of degrees experienced in some systems. In a military system, for example, a remotely operated laser may be turned on, after it has been at rest for some time, and may experience a temperature change from -50.degree.C. to its operating temperature of +40.degree.C. The laser must be frequency stabilized during warm up and during large environmental temperature PG,6 changes. A laser system using the device of the Midavaine patent requires a complex, expensive and heavy temperature stabilizing heat exchanger.
The present invention not only avoids problems of the Midavaine et al system, but also avoids problems of optical power signature sensing of prior art systems by providing a non-optical signal for interpretation of laser signature.
It is an object of the present invention to provide laser stabilization that avoids or minimizes abovementioned problems.